Mark Gatiss added: "Series four! At last ! It's always special to return to Sherlock but this time it's even more special as we're doing... a special! Not only that, we'll then be shooting three more episodes which will take Sherlock and John Watson into deeper and darker water than ever before.

"The whole Sherlock team are terrifically excited to return for four new adventures. The Game is on - again!"

Sue Vertue - Executive Producer for Hartswood Films - explained that it had "taken a little while to get [filming] dates sorted".

"None of the boys are exactly sitting back twiddling their thumbs but there was unanimous goodwill to make this work, so we're thrilled that 221b is going to be inhabited again," she said.

Benedict Cumberbatch and Martin Freeman will return to Sherlock for the special and new series.